More than 50% of drugs on the market target G protein-coupled receptors (GPCRs). Among the most important of these are drugs used to treat neurological disorders, such as pain relievers, antidepressants and anti-psychotics, as well drugs used for neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. The relatively recent discovery of a family of proteins called "regulator of G protein signaling" (RGS), that attenuate GPCR signals by increasing the GTPase activity of associated G proteins, has opened up a new avenue for modulating the activity of endogenous and administered GPCR ligands. However, development of inhibitors to RGS proteins has been hampered by the lack of robust assay methods for high throughput screening (HTS). In Phase I we developed fluorescence based GDP detection assays to measure RGS-dependent increases in the steady state GTPase activity of modified G proteins. The G proteins were mutated to increase the rate of GDP dissociation relative to GTP hydrolysis so that the GTPase accelerating activity of RGS proteins could be directly measured. These developments comprise the first HTS-compatible biochemical assay system for measuring RGS catalytic activity. In Phase II we will extend the approach to additional RGS and G proteins, and use the novel molecular tools in combination with cheminformatics to develop RGS-selective small molecule inhibitors. The inhibitors will be characterized biochemically and there in a cellular context. The availability of these novel HTS assays and inhibitors will accelerate drug discovery focused on RGS proteins and delineation of their roles in GPCR signal transduction. We will continue our successful collaboration with Dr. David Siderovski at the Department of Pharmacology, University of North Carolina Medical School in Phase II. PUBLIC HEALTH RELEVANCE: Over half of current drugs exert their effects through a family of proteins called G protein- coupled receptors, and the targeting of these receptors has been especially useful for the development of drugs used to treat neurological disorders such as schizophrenia, depression, and Parkinson's disease. To accelerate discovery of more selective therapies for these and other diseases, we are developing and validating novel screening assays for a family of proteins, called 'regulator of G-protein signaling'(RGS) that modulate the effects of GPCR ligands in a tissue-specific manner.